1. Field of Invention
The present invention relates to a steering wheel and, more specifically, to a steering wheel frame structure suitable for using materials such as magnesium or magnesium alloys for a core metal of the steering wheel.
2. Description of the Related Art
Motor vehicles are yearly improving in terms of safety. In keeping with this trend, steering wheels are being refined to provide new safety features. Steering wheels are conventionally adapted only to be operated for maneuvering motor cars. In recent years, however, cars are commonly equipped with an air bag device to protect a driver at the time of collision. Furthermore, steering wheels themselves are designed to absorb shocks during a collision. To that end, steering wheels are provided with a frame having a predetermined rigidity and yet providing cushion at the time of collision.
Generally, a steering wheel frame comprises a boss mounted to a steering shaft, a base frame supporting the boss, an annular-shaped grip frame, and a plurality of spoke frames extending from the base frame and securing the grip frame to the base frame. The positional relationship between the grip frame and the base frame can be described with reference to a truncated cone. The grip frame corresponds to an outer periphery of a bottom surface of the cone, and the base frame corresponds to an outer periphery of a top surface of the cone. The top surface periphery being the one with a smaller diameter.
With a steering wheel having three spoke frames, the respective spoke frames are disposed upward and away from a boss plate. In a reference steering condition of the steering wheel, wherein the vehicle is traveling in a straight path rather than turning, the first and second spoke frames extend in a substantially horizontal manner from the grip frame toward a center thereof, and have their inner ends connected to each other by means of a connection bar. Also, the inner ends of the first and second spoke frames, respectively, are connected to the boss plate by means of two connection pieces which extend in a direction substantially parallel to an axis of the steering shaft.
The third spoke frame extends from the grip frame toward a center thereof between the first and second spoke frames, and an inner end of the third spoke frame is connected to the inner ends of the first and second spoke frames, respectively, by means of two straight connection bars.
When the steering wheel is forced in a forward or reverse direction with at least a force of a predetermined magnitude, the connection pieces deform in order to absorb shocks on a driver. The longer the connection pieces, the larger amounts of deformation can occur and more shock can be absorbed.
A steering wheel frame can be assembled by welding together a plurality of steel materials. In recent years, a well known technique is used which constructs the frame by die casting aluminum. The use of aluminum or aluminum alloys or materials having a relatively small specific gravity is useful in making steering wheels lightweight. Since lightweight steering wheels are in demand, magnesium has been proposed as a lighter material for steering wheel construction.
However, when a steering wheel frame is formed by die casting, bent portions are formed between respective spoke frames and connection pieces and between a base plate and the connection pieces. Magnesium has a small specific gravity and a low ductility. Accordingly, when connection pieces of a steering wheel, are die casted of magnesium during a collision when the steering wheel deforms, bent portions of the connection pieces may break and accordingly not adequately absorb shocks applied to the steering wheel.